This application claims priority to prior Japanese applications JP 2003-106565 and 2003-170429, the disclosures of which are incorporated herein by reference.
The present invention relates to a chip-type solid electrolytic capacitor and a method of producing the same.
An existing solid electrolytic capacitor using tantalum as a valve metal is small in size, large in capacitance, and excellent in frequency characteristic and is widely used, for example, in a power supply circuit of a CPU (Central Processing Unit).
In order to further improve the frequency characteristic, development is made of a solid electrolytic capacitor in which a conductive polymer is used as a cathode layer instead of manganese dioxide so that an equivalent series resistance (ESR) is improved and reduced to {fraction (1/10)} or less.
However, as an operation frequency of the CPU becomes higher, there is an increasing demand for an improvement in noise characteristic of a power supply circuit for the CPU as well as an increase in allowable ripple current. As a consequence, a capacitor further lowered in ESR is required.
An apparatus to which the CPU is mounted is under development towards a small size and an advanced function. Accordingly, the solid electrolytic capacitor is required to simultaneously satisfy not only a lower ESR but also a small size, a large capacitance, and a thin profile.
Generally, if a plurality of capacitors are connected in parallel, a total capacitance Ctotal and a total equivalent series resistance ESRtotal are given by:
Ctotal=C1+C2+ . . . +Cnxe2x80x83xe2x80x83(1)
1/ESRtotal=1/ESR1+1/ESR2+ . . . +1/ESRnxe2x80x83xe2x80x83(2)
where Ci and ESRi represent a capacitance and an equivalent series resistance of an i-th capacitor (i=1, 2, . . . , n), respectively.
Therefore, if a plurality of capacitor elements can be connected in parallel within a limited space having a volume and a shape as desired, the capacitance will increased and the ESR will be decreased. This also applies to a case where the solid electrolytic capacitor is operated as a transmission-line noise filter.
The solid electrolytic capacitor comprising a plurality of capacitor elements connected in parallel is disclosed, for example, in Japanese Patent Application Publications No. H6-168854, H7-240351, and 2001-284192 which will hereinafter be referred to as Reference 1, Reference 2, and Reference 3, respectively.
Referring to FIG. 1, a multilayer solid electrolytic capacitor disclosed in Reference 1 includes a plurality of capacitor elements, an anode lead frame 271, a cathode terminal 272, a metal plate 273 with a protruding metal plate, and a reinforcing resin 274. Each of the capacitor elements comprises an anode metal foil 275, an insulator layer 276 formed at a predetermined position of the anode metal foil 275 to define an anode portion and a cathode portion, and a cathode member 277 formed in the cathode portion.
Referring to FIG. 2, an electrolytic capacitor disclosed in Reference 2 comprises a plurality of anode foils 281, a plurality of cathode foils 282, a plurality of cathode lead wires 282a, and an external anode terminal 285a, and an external cathode terminal 285b. 
Referring to FIG. 3, a solid electrolytic capacitor disclosed in Reference 3 comprises an anode lead frame 290, a pair of unit capacitor elements 292a and 292b, a cathode lead frame 293, and a pair of anode lead wires 295.
In case where a plurality of capacitor elements are connected in parallel so as to achieve capacitor having a small size and a thin profile as well as a lower ESR and a high capacitance, there arise several problems in a connection structure between a plurality of anode lead wires and a plurality of anode terminals.
For example, in the example described in Reference 1, connection of the anode lead frame requires the metal plates different in shape and the reinforcing resin. It is therefore difficult to reduce the number of steps in a production process.
In the example described in Reference 2, a plurality of the anode lead wires are welded at welding portions on the same side of the anode terminal in close proximity to one another. Therefore, adjacent ones of the welding portions may interfere with each other, resulting in frequent occurrence of variation in connecting strength and in electric characteristics. Furthermore, it is not easy to sufficiently lower an electric resistance at a welding portion between each of the anode lead wires and the anode terminal.
In the example described in Reference 3, the anode lead wire must be processed by machining before it is welded to the anode lead frame. It is therefore difficult to increase the reliability of connection between the anode lead frame and the anode lead wire. In case where the anode lead wire is bonded by the use of a silver paste, it is not easy to lower the electric resistance.
Thus, in the conventional solid electrolytic capacitors, the reliability of connection and the electric characteristics tend to be varied as a result of an asymmetrical structure of the connecting portion between the anode lead wires and the anode terminal. It is therefore difficult to lower a production cost. Furthermore, if a plurality of welded portions are located on the same side of the anode terminal in close proximity to one another, adjacent ones of the welded portions interfere with each other. This results in easy occurrence of variation in connecting strength and in electric characteristics.
It is therefore an object of the present invention to provide a chip-type solid electrolytic capacitor which is low in ESR, high in capacitance, and excellent in reliability.
It is another object of the present invention to provide a method of producing a chip-type solid electrolytic capacitor mentioned above.
Other objects of the present invention will become clear as the description proceeds.
According to an aspect of the present invention, there is provided a chip-type solid electrolytic capacitor having a mounting surface and comprising a pair of capacitor elements laminated in a predetermined direction perpendicular to the mounting surface, each of the capacitor elements using a valve metal and having an anode member and a cathode layer mechanically coupled to the anode member, a pair of anode lead wires each of which is led out from the anode member in parallel to the mounding surface, an anode terminal connected to the anode lead wires, a cathode terminal connected to the cathode layer, and an encapsulating resin encapsulating the capacitor elements with the anode and the cathode terminals partially exposed, the anode terminal including two branches having branch end portions, respectively, which are formed by shaping, the branch end portions having shapes substantially same to each other so that the branch end portions overlap each other by rotation of 180xc2x0 around a straight line at an intermediate position between the anode lead wires, the branch end portions being welded to the anode lead wires to produce welded portions, respectively.
According to another aspect of the present invention, there is provided a chip-type solid electrolytic capacitor having a mounting surface and comprising three capacitor elements laminated in a predetermined direction perpendicular to the mounting surface, each of the capacitor elements using a valve metal and having an anode member and a cathode layer mechanically coupled to the anode member, three anode lead wires each of which is led out from the anode member in parallel to the mounding surface, an anode terminal connected to the anode lead wires, a cathode terminal connected to the cathode layer, and an encapsulating resin encapsulating the capacitor elements with the anode and the cathode terminals partially exposed, the anode terminal including three branches having a first, a second, and a third branch end portion, respectively, which are formed by shaping, the first and the third branch end portions having shapes so as to overlap each other by rotation of 180xc2x0 around a straight line, the second branch end portion being between the first and the third branch end portions, the first, the second, and the third branch end portions being welded to the anode lead wires to produce welded portions, respectively.
According to sill another aspect of the present invention, there is provided a chip-type solid electrolytic capacitor having a mounting surface and comprising four capacitor elements laminated in a predetermined direction perpendicular to the mounting surface, each of the capacitor elements using a valve metal and having an anode member and a cathode layer mechanically coupled to the anode member, four anode lead wires each of which is led out from the anode member in parallel to the mounding surface, an anode terminal connected to the anode lead wires, a cathode terminal connected to the cathode layer, and an encapsulating resin encapsulating the capacitor elements with the anode and the cathode terminals partially exposed, the anode terminal including four branches having a first, a second, a third, and a fourth branch end portion, respectively, which are formed by shaping, the first and the fourth branch end portions having shapes so as to overlap each other by rotation of 180xc2x0 around a straight line, the second and the third branch end portions being between the first and the fourth branch end portions and having shapes so as to overlap each other by rotation of 180xc2x0 around the straight line, the first, the second, the third, and the fourth branch end portions being welded to the anode lead wires to produce welded portions, respectively.
According to yet another aspect of the present invention, there is provided a method of producing a chip-type solid electrolytic capacitor having a mounding surface and comprising a plurality of capacitor elements which are laminated in a direction perpendicular to the mounting surface and electrically connected in parallel, each of the capacitor elements having an anode member using a valve metal, an anode lead wire led out from the anode member, and a cathode layer mechanically coupled to the anode member. The method comprises the steps of preparing a lead frame having an anode terminal forming portion and a cathode terminal forming portion, the anode terminal having a center line and a plurality of branches symmetrical with each other with respect to the center line, shaping the branches by bending, welding the capacitor elements to the anode terminal forming portion and connecting the cathode layer to the cathode terminal forming portion, molding the capacitor elements, connected to the lead frame, by an encapsulating resin to obtain a molded body, and cutting the lead frame to separate a part of the lead frame from the molded body.
According to a further aspect of the present invention, there is provided a chip-type solid electrolytic capacitor having a mounting surface and comprising a capacitor portion having a plurality of capacitor elements laminated in a predetermined direction perpendicular to the mounting surface, each of the capacitor elements using a valve metal, a cathode terminal led out from the capacitor portion, a plurality of anode lead wires led out from the capacitor portion in parallel to the mounting surface, the anode lead wires being positioned apart from each other in the predetermined direction, an anode terminal connected to the anode lead wires; and an encapsulating resin encapsulating the capacitor portion with the anode and the cathode terminals partially exposed, the anode terminal including a plurality of branch end portions which are positioned apart from each other in the predetermined direction and welded to the anode lead wires, respectively.